Anisotropic (directional) dry etching is conventionally used to selectively remove horizontal portions of a film, while leaving substantially vertical portions of the film intact. For example, reactive ion etching (RIE) is used to create sidewalls (or "spacers") on semiconductor structures such as gate stacks. (See U.S. Pat. No. 4,256,514 to Pogge).
Selective removal of substantially vertical portions of a film while leaving horizontal portions intact is a different issue altogether, and is desirable in order to create features with sub-micron lateral dimensions equal to the thickness of the vertical film portion. Existing techniques include masking of the horizontal portions followed by (1) isotropic (non-directional) wet dips which can undercut either portions of the film to be kept or adjacent structures; (2) isotropic dry film removal techniques such as parallel plate plasma processing and chemical downstream plasma reactions which can result in undercut problems as well; or (3) directional dry etch processes (RIE) which can result in incomplete removal of the vertical portion, which requires extensive overetch or a subsequent wet dip to remove. In the last technique, extensive overetch can cause excessive resist erosion, degradation of profile integrity, or erosion of the etch stop material.
The above processes all use masking to protect the horizontal portions of the film, and as such add extra process steps and thus introduce sources of process error. One method that does not use a mask is described in U.S. Pat. No. 4,801,544 to Gobrecht, et al. Gobrecht deposits a non-conformal film with thicker horizontal portions and thinner vertical portions, followed by non-selective etching to remove the vertical portion first and leave part of the horizontal region. However, controlling both the deposition and the etching to remove a vertical film portion of desired thickness is difficult, a "lobe" will tend to occur on the corner, and the non-selective etching is likely to remove some of the horizontal portion immediately adjacent to the vertical portion.
Thus, there remains a need to remove the substantially vertical portions of a film without significantly attacking non-masked horizontal portions of the same film, and at the same time avoid attacking adjacent films or structures in a simplified, repeatable process.